Expression of herpes simplex virus thymidine kinase, mediated by vaccinia virus early promoters

We measured herpes virus thymidine kinase (HSV-TK) activity in extracts from cells infected with eight vaccinia virus recombinants (VpT), each expressing HSV-TK under the control of an early promoter previously isolated by a shotgun procedure. The HSV-TK activities induced by the VpT recombinants were compared to that produced under the control of the vaccinia virus thymidine kinase (VV-TK) promoter in VMM5-TK recombinant virus. The insert from VpT38 was approximately 10 times more efficient than the VV-TK promoter for HSV-TK expression, reflecting a similar relative strength of the promoters. HSV-TK activities induced by the other VpT recombinants varied between one and ten times that expressed by the VV-TK promoter, but the nucleotide sequence of the 5'-end region of their mRNA suggested that these values did not necessarily reflect the strength of corresponding promoters. No significant reduction in HSV-TK activity was noted for two VpT and the VMM5-TK recombinants when viral DNA replication was prevented, but a significant reduction (30 to 75%) was observed for the other six recombinants studied. These results suggested that some early genes of vaccinia virus are expressed only during the early stage of infection, whereas others continue to be expressed at the late stage. The strength of two vaccinia early promoters (VpT38, PF) relative to that of the VV-TK promoter was deduced from HSV-TK activities induced by comparable vaccinia virus recombinants.